fix: add showingError bool to editor header

Source/Plugin/PluginBridgeEditor.h

@@ -30,6 +30,9 @@ private:
     // Embedded plugin editor
     std::unique_ptr<juce::AudioProcessorEditor> hostedEditor;
 
+    // State
+    bool showingError = false;
+
     static constexpr int kTopBarHeight = 40;
     static constexpr int kDefaultWidth = 400;
     static constexpr int kDefaultHeight = 60;